My research focuses on atoms in which one electron is excited to a state of very large principal quantum number n, termed Rydberg atoms. Such atoms possess properties quite unlike those normally associated with atoms in ground or low-lying excited states.For example, because the size of an atom scales as n2, they are physically very large.An atom with n~400 has a diameter approaching 20 μ-m.Their classical electron orbital periods are also very long, ~10 ns at n~400, and this we exploit to control and manipulate their electronic wave functions using pulsed electric fields whose characteristic times (duration, rise/fall times) are less than this period. Application of such pulses leads to the formation of wave packets that comprise a coherent superposition of adjacent Rydberg states. These wave packets display novel dynamical behavior that can mimic the classical motion of an excited electron, thereby providing a bridge between quantum and classical physics. Our studies show that, with careful choice of the amplitude and width of the pulses (or application of several pulses), it is possible to control and manipulate atomic wavefunctions with remarkable precision.The goal is to use carefully-tailored sequences of pulses to engineer atomic wavefunctions and produce “designer” atoms to study classical-quantum correspondence, to use periodic trains of pulses to create non-dispersive wave packets and to probe non-linear dynamics and classical/quantum chaos, to study atom/field interactions in the ultra-fast ultra-intense regime, to explore information storage/retrieval in atoms, and to study the behavior of strongly-coupled Rydberg/Rydberg systems.Initial work centered on potassium Rydberg atoms contained in a tightly-collimated atomic beam but has now been extended to include strontium Rydberg atoms contained in both an atomic beam and in an ultra-cold atomic gas or Bose-Einstein condensate (BEC).

The starting point for many of our studies is the production of strongly-polarized quasi-one-dimensional (quasi-1D) high-n Rydberg atoms by photo-exciting extreme red-shifted Stark states in the presence of a weak dc field.Protocols have been developed using pulsed electric fields to characterize such states and monitor their time evolution.Techniques have also been devised to convert such states into quasi-two-dimensional (quasi-2D) “circular” states in which the electron orbits in a plane, and probe their evolution.The opportunities afforded by the creation of quasi-1D and -2D atoms are being explored.These include the use of periodic drive fields to create localized non-dispersive wave packets that mimic the behavior of a classical electron and to transport these wave packets to states of higher n.Measurements with strontium Rydberg atoms show that, even at high n, the regime where dipole blockade becomes important can be accessed opening up new opportunities to study strongly-coupled Rydberg atom pairs and their possible manipulation to form long-lived “molecular” species through periodic driving.The production of long-lived “planetary atom” states in strontium that contain two excited electrons is also being examined.The evolution of cold Rydberg gases into a strongly-coupled ultracold neutral plasma through collisions is also under investigation.Studies of the control of the interactions between atoms in a strontium BEC by dressing the ground-state atoms with an admixture of a higher-lying Rydberg state using lasers have been initiated with the long-range goal of realizing soliton formation in three dimensions and creation of a supersolid

Because the Rydberg electron is typically far from its associated core ion it will, in collisions with neutral targets, behave as an independent particle allowing a Rydberg atom to be viewed as an ultra-low-energy electron trap.Potassium Rydberg atoms are therefore being used to examine electron attachment to molecules.The creation of novel negative ion species in such collisions is observed including the formation of negative ions in which the electron is weakly bound by the electric dipole moment of the parent molecule.The creation of heavy-Rydberg ion-pair states comprising a positive-negative ion pair that orbit at large radius weakly bound by their mutual electrostatic attraction is also seen.The physical and chemical properties of such species are being examined using a newly-commissioned apparatus.

Each of the above experimental programs makes use of advanced experimental techniques including ultrahigh vacuum technology, state-of-the-art dye and diode laser systems, high-speed analog and digital electronics, and computer-based data acquisition and control systems.The work relies heavily on the efforts of several graduate students:

Changhao Wang

Michael Kelley

Xinyue Zhang

Sitti Buathong

Roger Ding

Gavin Fields

Research Topics

Protocols to create localized wave packets in very-high-n (n ~ 300) states that travel in near circular orbits about the nucleus are being developed. These wave packets represent the closest analog yet achieved to the original Bohr model of the hydrogen atom, i.e., an electron in circular classical orbit about the nucleus. Initially, quasi-one-dimensional Rydberg atoms strongly polarized along the x axis are created by photoexciting the lowest-lying states in the n ~ 306 Stark manifold in the presence of a weak dc field, directed along the x axis. This is then turned off adiabatically and a strong transverse field is suddenly applied along the y axis. This leads to the creation of a Stark wave packet which undergoes periodic changes in its y component of angular momentum. Turn-off of the transverse field at an appropriate time leaves a localized wave packet traveling in a near-circular orbit (n ~ l ~ m) that undergoes transient localization forming a Bohr-like state that remains localized for several orbits before dephasing and collapsing to a quasi-stationary near-circular state. The use of radio frequency fields to maintain strongly localized Bohr-like wave packets for many hundreds of orbits is being explored.With careful choice of the period, amplitude, and phase of the drive field wave packets have been generated, termed Trojan wave packets that execute near-circular motion and that remain localized indefinitely and whose motion mimics that of Jupiter's Trojan asteroids.Furthermore, the locking to the drive field is so strong that by slowly varying the drive frequency the wave packet can be transported to states of much higher n, furnishing a new technique for the control of atomic wavefunctions.This work is being extended using strontium to explore the creation of quasi-stable planetary atoms containing two excited electrons.

Schemes for the production of quasi-1D high-n Rydberg atoms at densities in the blockade regime where the excitation of one atom inhibits the subsequent excitation of its neighbors are being explored as this allows the controlled production of Rydberg atoms with well-defined initial separations.At n~300, creation of a quasi-1D atom, for an effective laser linewidth of ~5 MHz, suppress excitation of a second similar atom within a radius of ~100 μ-m extending strong many-body interactions well into the mesoscopic regime.Measurements using two- and three-photon excitation of strontium atoms in a beam show that it is possible to achieve the required densities.Access to the dipole blockade regime will enable detailed studies of the interactions between strongly-coupled Rydberg atoms and their dependence on the coupling strength, which can be varied by changing n or by using the techniques we have already developed to manipulate the atomic state.Questions of interest focus on the dynamics of energy interchange and their dependence on the degree of coupling.Excitation to Rydberg states whose size is comparable to the interatomic spacing will lead to the formation of a transient Rydberg “molecule” whose stability against autoionization is governed by electron-electron scattering.Of particular interest are long-lived configurations where, due to their correlated motions, the electrons remain far apart.Such states might also, through periodic driving with an electric field lead to formation of a correlated phase-locked Rydberg pair wave packet.

In other experiments strong tunable long-range interactions are being created in cold atom samples and BECs by dressing ground state strontium atoms contained in an optical dipole trapwith an admixture of an excited Rydberg levelusing laser radiation tuned near resonance with the transition to the Rydberg state.Strontium possesses narrow intercombination lines which can be used to obtain significantly longer sample lifetimes and decoherence times as compared to the use of alkali atoms.These ongoing experiments will advance our understanding of quantum materials and emergent phenomena and lead to new technological advances. Initial measurements, however, have revealed that even weak Rydberg dressing can lead to surprisingly large ground state atom loss rates from the trap. The processes responsible continue to be examined but are attributed to the excitation of Rydberg atoms which can result in direct trap loss through recoil and in the population of very-long-lived metastable states. Measurements of the time dependence of the trap loss suggest a mechanism in which the creation of a small number of “seed” Rydberg atoms triggers “avalanche-like” growth in the numbers of Rydberg atoms excited. This work shows that, if effects due to Rydberg dressing are to be observed, small samples and/or short time scales may be essential. To this end, the use of Autler-Townes spectroscopy has been exploited to study the effects of interactions induced by Rydberg dressing on timescales of a few microseconds. The effects of interactions are observed in shifts, asymmetries, and broadening of the measured atom loss spectra. The experiment is analyzed using a density matrix approach that accounts for interaction-induced level shifts and dephasing and incorporates the effects of Rydberg blockade. The model provides good agreement with experiment for short dressing times but for longer dressing times breaks down, suggesting that additional sources of dephasing become important which continue to be investigated.

Spectroscopic measurements of trap loss also revealed the production of very-long-range weakly-bound Rydberg molecules in which scattering of the excited Rydberg atom from a neighboring ground state atom can bind the two atoms together. Detailed studies of such molecules are underway as they represent a new type of chemical bond and can display surprising features such as the presence of large permanent electric dipole moments, even in the case of a homonuclear molecule. Measurements reveal the production of Rydberg molecules in both ground and excited vibrational states whose energies are in excellent agreement with those predicted theoretically. The data also show the presence of features that result from the creation of trimer (tetramer) states that comprise a parent Rydberg atom and two (three) bound atoms and whose energies are given approximately by the separate binding energies of the atoms. Measurements on these novel species are continuing to compare their properties (lifetimes, electric field ionization characteristics, etc.) to those of the parent Rydberg atoms.

Collisions between high-n Rydberg atoms and molecular targets are being used to examine electron attachment to molecules. The creation of novel negative ion species is observed including the formation of ions in which the electron is weakly bound by the electric dipole moment of the parent molecule. Such ions display many of the characteristics associated with Rydberg atoms which is, perhaps, not unexpected as they both contain a weakly-bound electron in a diffuse orbital. Rydberg atom collisions can also lead to creation of so-called heavy-Rydberg ion-pair states comprising a positive-negative ion pair that orbit at large radius weakly bound by their mutual Coulomb attraction. The lifetimes and decay modes of such species are being examined. Ion-pair states involving molecular negative ions can decay through mutual charge transfer or the conversion of internal energy in the negative ion into translational energy of the ion pair. Those states involving atomic negative ions have very long lifetimes, allowing studies of their chemical and collisional properties.

Each of the above experimental programs makes use of advanced experimental techniques including ultrahigh vacuum technology, state-of-the-art dye, diode and fiber laser systems, high-speed analog and digital electronics, and computer-based data acquisition and control systems. The work relies heavily on the efforts of several graduate students:

"Generation of localized "Bohr-like" wavepackets in near-circular orbit about the nucleus." Division of Atomic, Molecular, and Optical Physics of the American Physical Society, University of Virginia, Charlottesville, Virginia. (May, 2009)

"Photoexcitation of high-n, n~300, Rydberg states in the presence of an rf driving field near the final Kepler frequency." 43rd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics, Anaheim, California. (June 2012) With S. Ye, X. Zhang, S. Yoshida, and J. Burgdörfer

"Properties of heavy Rydberg ion-pair states formed in collisions between K(np) Rydberg atoms and attaching targets." 2010 Annual Meeting of the Division of Atomic, Molecular, and Optical Physics of the American Physical Society, Houston, TX. (May 2010) With M. Cannon, C. H. Wang

The influence of stray fields on the ionization of Rydberg atoms at metallic surfaces - 2010 Annual meeting of the Division of Atomic, Molecular, and Optical Physics of the American Physical Society, Houston, TX (with Dennis Neufeld and Yu Pu)

"Formation of heavy Rydberg ion-pair states in collisions of K(np) Rydberg atoms with attaching targets." 40th Annual Meeting of the Division of Atomic, Molecular, and Optical Physics of the American Physical Society, University of Virginia, Charlottesville, Virginia. (July 2009) With M. Cannon, C. Wang

"Ionization of Rydberg atoms at metal surfaces." 40th Annual Meeting of the Division of Atomic, Molecular, and Optical Physics of the American Physical Society, University of Virginia, Charlottesville, Virginia. (May 2009) With Dennis Neufeld, Yu Pu